Plasticized polyvinyl butyral sheet containing epoxy resin

ABSTRACT

Plasticized polyvinyl butyral sheet containing epoxy resin in amount effective, after prolonged exposure to light, to counteract reduction of adhesion between the sheet and a photoreactive component with which it will be in potential contact. The photoreactive component is typically a dielectric such as a metal oxide layer of a heat-wave-reflective or electrically conductive multi-layer coating, which coating can optionally be supported on (a) a thermoplastic substrate sandwiched between two of such polyvinyl butyral sheets forming a prelaminate for use as a constituent of a laminated glazing panel or (b) a glass sheet component of such glazing panel.

This is a DIVISION of copending application Ser. No. 08/088,147, filedJul. 1, 1993 now pending.

BACKGROUND OF THE INVENTION

This invention relates to polyvinyl butyral sheet forlight-transmitting, layered safety glazing panels and more particularlyto such sheet formulated to improve performance when aheat-wave-reflective or electrically conductive coating is included insuch panels.

Light transmitting safety glazings for window, windshield, sunroof,skylight, intrusion security, showcase, picture frame and likeapplications are well-known. They include one or more rigid transparentpanels such as glass combined in a laminate with an impact-dissipatingplastic sheet such as plasticized polyvinyl butyral (PVB). It islikewise known to control the strength of the bond between the plasticsheet and rigid panel at a desired level since if too high the plasticsheet undesirably ruptures on impact and if too low, splinters of therigid panel can separate from the glazing, and if glass, can injure aperson in the surrounding area.

It is further known to incorporate heat-wave-reflective multi-layercoatings (sometimes called "stacks") into such safety glazings toreflect infra-red radiation while transmitting significant visiblelight. The effect is to reduce temperature increase from solar radiationwithin an area delimited by one or more of such safety glazing panels.Heating a metal layer of such multi-layer coatings by electricalconductance reduces the time required for defrosting or defogging.Representative structures for vehicle windshields are disclosed in U.S.Pat. Nos. 4,799,745 and 4,782,216. In such safety glazings, theinitially exposed, uncovered top layer of the multi-layer coating oftencontacts the plasticized PVB sheet. Such top coating layer is oftenphotosensitive in that after extended exposure to light, the initiallystrong bond between the sheet and top coating layer deteriorates.

As disclosed in U.S. Pat. No. 5,061,568 and European Patent Application263623 published Apr. 13, 1988, this adhesion durability problem hasbeen addressed by proposing a special cap layer on theheat-wave-reflective coating to contact the plasticized PVB, which caplayer is chosen for its capability to preserve the bond under theinfluence of light. Though overcoming the adhesion problem, such caplayers are usually deposited by an additional process sputtering stepwhich can be slow and therefore costly to the total value of the safetyglazing panel.

SUMMARY OF THE INVENTION

Now improvements have been made in light-resistant adhesion ofplasticized PVB sheet to photoreactive components of laminated glazingpanels which reduce shortcomings of the prior art by avoiding relianceon sputtered cap layers.

Accordingly, a principal object of this invention is to stabilize theadhesion between plasticized PVB sheet and a photoreactive component(s)of a laminated glazing panel.

Another object is to provide light-resistant adhesion betweenplasticized PVB sheet and a dielectric layer of a heat-wave-reflectiveor electrically conductive coating of a laminated glazing panel.

A specific object is to improve the adhesion stability of the bondbetween plasticized PVB sheet and a heat-wave reflective or electricallyconductive coating on glass.

A further object is to enhance such long-term adhesion by modifying theformulation of the plasticized PVB sheet.

Other objects will in part be obvious and will in part appear from thefollowing description and claims.

These and other objects are accomplished by providing plasticizedpolyvinyl butyral sheet containing epoxy resin in amount effective,after prolonged exposure to light, to counteract reduction of adhesionbetween the sheet and a photoreactive component with which it will be inpotential contact.

Also provided is plasticized polyvinyl butyral sheet containing epoxyresin bonded to a photoreactive layer of a heat-wave-reflective orelectrically conductive coating, the epoxy resin present in amounteffective, after prolonged exposure to light, to counteract reduction ofadhesion at the bond interface.

Further provided is a prelaminate for a glazing panel having improvedresistance to light exposure comprising two plasticized polyvinylbutyral sheets and an interposed intermediate layer supported on athermoplastic substrate, the intermediate layer containing aphotoreactive layer bonded to one of the plasticized polyvinyl butyralsheets, the plasticized polyvinyl butyral sheet bonded to thephotoreactive layer comprising one or more epoxy resins in amounteffective to counteract reduction of adhesion to the photoreactive layerafter prolonged exposure to light.

Also provided is a laminated glazing panel sequentially comprising: (A)a glass sheet; (B) a heat-wave-reflective or electrically conductivemulti-layer coating which includes a photoreactive layer; and (C) asheet of plasticized polyvinyl butyral in interfacial contact with thephotoreactive layer containing epoxy resin in amount effective tocounteract reduction of adhesion between the sheet and the photoreactivelayer after prolonged exposure of the glazing panel to light.

DETAILED DESCRIPTION OF THE INVENTION

In addition to stabilizing adhesion to photoreactive component(s) of aheat-wave-reflective or electrically conductive coating after prolongedexposure to light, epoxy resin usable in the invention, atconcentrations described below, should have no significant adverseeffect on processing plasticized PVB resin or on performance propertiesof the sheet of which it is a component. In this latter respect, usableepoxy resins: a) should readily mix with components of formulations ofconventional plasticized PVB sheet for laminated safety glass, i.e. PVBresin, PVB plasticizer, glass adhesion control and relatedproperty-enhancing additives; b) should be melt processable in suchformulations in conventional equipment such as extruders, sheeting dies,mill rolls and the like; and c) as a component of sheet which on oneside will always contact a glass pane of a laminated safety glazing,have little or no affect on adhesion to glass (as opposed to thephotoreactive component), such glass adhesion being regulated by glassadhesion-reducing agent in the sheet formulation. To determine whetheran epoxy is functional as an adhesion-stabilizer in the context firstmentioned above, the pane of glass of a glass laminate which is nearestthe bond between the photoreactive component and the plasticized PVBsheet containing the epoxy resin must display a pummel adhesion(described further below) of at least 3.0 after at least 1000 hoursexposure in a Weatherometer, Fadeometer or equivalent accelerated agingexposure system. By sufficient preliminary tests, one skilled in the artcan readily determine particular usable epoxy resins. With the foregoingcriteria in mind, usable epoxy resins may vary in chemical identity.Preferred epoxy compositions found usable as hereinafter described areselected from (a) epoxy resins comprised mainly of the monomericdiglycidyl ether of bisphenol-A; (b) epoxy resins comprised mainly ofthe monomeric diglycidyl ether of bisphenol-F; (c) epoxy resinscomprised mainly of the hydrogenated diglycidyl ether of bisphenol-A;(d) polyepoxidized phenol novolacs; (e) diepoxides of polyglycols,alternatively known as an epoxy terminated polyether; and (f) a mixtureof any of the foregoing epoxy resins of (a) through (e). To saveunnecessarily detailed description, further information on these classesis in the Encyclopedia of Polymer Science and Technology, Volume 6,1967, Interscience Publishers, N.Y., pages 209-271, which isincorporated herein by reference.

A suitable commercially available diglycidyl ether of bisphenol-A ofclass (a) is DER 331 from Dow Chemical Company. A diglycidyl ether ofbisphenol-F epoxy of class (b) is EPON Resin DPL-862 and a hydrogenateddiglycidyl ether of bisphenol-A epoxy of class (c) is EPONEX Resin 1510,both of the latter available from Shell Chemical Company. APolyepoxidized phenol formaldehyde novolac of class (d) is availablefrom Dow Chemical as DEN 431. A diepoxide of Poly(oxypropylene) glycolof class (e) is available from Dow Chemical as DER 732.

Epoxy resins found unsuitable as adhesion promoters for this applicationare epoxidized soybean oil and octyl epoxy tallate. Though presentlylacking experimental evidence, it is postulated that the functionalepoxy groups in these latter resins are situated between hydrocarbongroups of varying lengths, thereby hindering access of the epoxy groupsto the photoreactive layer of the heat-wave reflective coatings. This iscontrasted with the successful epoxies noted above where epoxy groupsare at the ends of the molecules and therefore thought to be readilyaccessible for association with the photoreactive layer of the heat-wavereflective coating.

PVB resin in sheet of the invention has a weight average molecularweight greater than 100,000, preferably from about 200,000 to 300,000,as measured by size exclusion chromatography using low angle laser lightscattering. Such PVB comprises, on a weight basis, 15 to 25%, preferably18 to 22% hydroxyl groups calculated as polyvinyl alcohol (PVOH); 0 to10%, preferably 0 to 3% residual ester groups, calculated as polyvinylester, e.g. acetate, with the balance being butyraldehyde acetal.

PVB resin is produced by known aqueous or solvent acetalization byreacting butyraldehyde with PVOH in the presence of acid catalyst toproduce PVB, followed by neutralization of the catalyst, separation,stabilization and drying of the resin. It is available from MonsantoCompany as Butvar® resin.

In forming sheet, the PVB resin must be plasticized with about 20 to 80and more commonly 25 to 45 parts plasticizer per hundred parts of resin.The latter concentration is generally used with PVB containing 18 to 22%vinyl alcohol by weight. Plasticizers commonly employed are esters of apolybasic acid or a polyhydric alcohol. Suitable plasticizers aretrimethylene glycol di-(2-ethylbutyrate), dihexyl adipate, dioctyladipate, mixtures of heptyl and nonyl adipates, dibutyl sebacate,polymeric plasticizers such as the oil-modified sebacic alkyds, andmixtures of phosphates and adipates such as disclosed in U.S. Pat. No.3,841,890 and adipates and alkyl benzyl phthalates as disclosed in U.S.Pat. No. 4,144,217. Also mixed adipates made from C₄ to C₉ alkylalcholos and cyclo C₄ to C₁₀ alcohols as disclosed in U.S. Pat. No.5,013,779. C₆ -C₈ adipate esters such as hexyl adipate are preferredplasticizers.

Plasticized PVB sheet about 0.13 to 1.3 mm thick for adequate impactabsorption in a laminate is formed by initially mixing the PVB resin,plasticizer, and epoxy resin to form a sheet formulation and thenextruding the formulation, usually through a sheeting die, i.e. forcingthe molten, plasticized PVB through a horizontally long, verticallynarrow die opening substantially conforming in length and width to thatof the sheet being formed. Alternatively such sheet may be formed bycasting the molten formulation issuing from an extrusion die onto aspecially prepared surface of a die roll turning in close proximity tothe die exit to impart desired surface characteristics to one side ofthe molten formulation. When the roll surface has minute peaks andvalleys, sheet formed of polymer cast thereon will have a rough surfaceon the side contacting the roll which generally conforms respectively tosuch valleys and peaks. A rough surface on the other side can beprovided by the design of the die opening through which the extrudatepasses. Such a die opening configuration is more particularly shown inFIG. 4 of U.S. Pat. No. 4,281,980. Alternative known techniques ofproducing a rough surface on one or both sides of an extruding sheetinvolve the specification and control of one or more of the following:polymer molecular weight distribution, water content and temperature ofthe melt. Such techniques are disclosed in U.S. Pat. Nos. 2,904,844;2,909,810; 3,994,654; 4,575,540 and European Patent No. 0185,863. As isknown, this rough surface is temporary to particularly facilitatedeairing during laminating after which it is melted smooth from theelevated temperature and pressure associated with autoclaving, in whichstate it is optically transparent.

Glass adhesion control agents are included in the sheet formulation toreduce the strength of the bond between contacting surfaces of the epoxyresin modified PVB sheet and a glass layer of the final laminated safetyglazing. Such glass-contacting PVB sheet surface in the structures to bepresently described is the PVB sheet side opposite the sheet sidecontacting the photoreactive component. Glass adhesion control (i.e.reducing) agents are known and typically disclosed in U.S. Pat. Nos.3,249,487; 3,855,055; 3,249,488; 4,292,372; 4,379,116; 3,402,099;3,371,235 and 4,180,620, the PVB additive disclosure of each of which isincorporated herein by reference. Preferred glass adhesion controlagents include monovalent and multivalent, for example divalent, metalsalts of C₁ to C₈ organic, preferably aliphatic, monocarboxylic acidswhere, for example, the metal cation is potassium, magnesium, calcium,sodium or zinc. Representative anions are acetate, butyrate, substitutedbutyrates such as 2-ethyl butyrate, octanoate etc. Magnesium 2-ethylbutyrate is a preferred glass adhesion control agent. The concentrationof glass adhesion control agent in the sheet for the desired level ofglass adhesion will generally be about 0.01 to 0.1 (preferably 0.01 to0.05) weight percent based on PVB resin or 100 to 500 parts, preferably200 to 400 parts by weight per million parts of combined plasticizer,PVB resin, epoxy resin and other additive(s) (if any).

The concentration of epoxy resin in the sheet should be adequate afterprolonged exposure of the sheet to light (further described hereafter)to preserve the bond between the sheet and photoreactive component(s) ata bond strength which provides the desired impact performance in thefinal laminated glazing panel while, as noted above, not adverselyaffecting the desired bond of the sheet with glass. In this regard, justas an artisan is concerned with glass adhesion not being too high orlow, so also in this invention the bond with the photosensitivecomponent(s) after prolonged exposure to light cannot be too high or lowto avoid such bond controlling the overall impact performance of theglazing. For example, concentrations on the order of 20 weight percentepoxy (based on PVB resin) in the sheet provide far too high adhesion toglass whereas 1 phr of epoxy is inadequate to provide any noticeableeffect on adhesion to photoreactive dielectric layers after long termlight exposure. Concentrations of about 3 to about 10, preferably about4 to 7 parts epoxy resin per 100 parts PVB resin in the sheet havegenerally been found to provide the desired balance of properties.

In addition to plasticizer, epoxy resin and glass adhesion-reducingagent(s) the PVB sheet of the invention may contain otherperformance-enhancing additives such as pigments or dyes, lightstabilizers, anti-oxidants and the like.

The epoxy resin modified plasticized PVB sheet of the invention isdesigned for laminating contact with a component sufficientlyphotoreactive after prolonged exposure to light, e.g. UV radiation fromthe solar spectrum, as to unacceptably deteriorate the strength of theinterfacial bond between the photoreactive component and conventionalunmodified plasticized PVB sheet. Such photoreactive component can varyin identity and can typically be any functional member or layer orcoating included in a safety glazing to improve its performance. It isusually the uppermost exposed layer of a multi-layer, heat-wavereflective or electrically conductive coating comprising a stack ofsuccessively deposited layers of metals, metal (usually dielectric)compounds and the like on a substrate. Such metal/dielectric stacks arecalled optical interference filters of the Fabry-Perot type designed,principally through the appropriate selection of materials and theirthicknesses to maximize (i) transmission of visible or luminous and (ii)reflection of heat-generating infra-red (IR) portions (700-2125 nm) ofthe solar spectrum. The multiple, sequentially deposited planar layersof angstroms-thick metal and dielectric layers are arranged in apredetermined sequence in face-adhering, contiguous contact with eachother, as generally disclosed in U.S. Pat. Nos. 3,682,528 and 4,179,181which are incorporated herein by reference.

Heat-wave reflective coatings are usable in any optically transparentwindow application. Typical applications include aircraft, locomotiveand automotive windshields and architectural applications such ascommercial and residential buildings. By conductively associating themetal layer(s) with a source of electrical power, usually through theuse of bus bars, defrosting or defogging or deicing capability isprovided in the assembly.

Preferred heat-wave reflective coatings contain at least two near IRreflecting metal underlayers which transmit at least 70% visible lightof normal incidence measured as specified in ANSI Z26.1, this being theminimum required in the U.S. automotive industry. Somewhat less isacceptable in less demanding architectural applications where a singlemetal layer or other more light absorbing metal/dielectric stacks may beused. The metal layer(s) are separated vertically in the thicknessdirection from each other by one or more dielectric layers so reflectionof visible light from the metal layer(s) interferes destructivelythereby enhancing visible transmission. Usable metals comprise silver,aluminum, chromium, zinc, tin, nickel, brass, gold, stainless steel,copper, and alloys or claddings of any of the foregoing. Silver at athickness of between 60 to 200, preferably 80 to 140 Angstroms, ispreferred.

The dielectric layer must be essentially transparent over the visiblerange and at least one must exist between a pair of metal layers (calleda "spacer dielectric layer") and preferably a dielectric layer is oneach side of a metal layer. Usable dielectric materials include WO₃, In₂O₃, SnO₂, ITO, Al₂ O₃, MgF₂, ZnS, TiO₂, ZnO and the like.

Varying the thickness and composition of a spacer dielectric layervaries the optical transmittance/reflection properties considerably.More specifically, varying the thickness of a spacer dielectric layervaries the wave length associated with the reflection suppression (ortransmission enhancement) band. In addition to choice of metal,thickness also determines its reflectivity, the thinner the layer, theless its reflectivity. Generally, the thickness of spacer dielectriclayer(s) should be about 200 to 1200, preferably 450 to 1000 A, toobtain the desired optical properties in a commercially acceptableproduct.

Exterior dielectric layers contacting metal layer surfaces opposite tometal surfaces contacting spacer dielectric layer(s) enhanceanti-reflection performance. Exterior dielectric layers generally shouldhave a higher refractive index than glass or polyvinyl butyral, i.e.greater than 1.5 and preferably greater than 1.8. The thickness of suchexterior or outside dielectric layer(s) is generally less than spacerdielectric layer(s) and should be about 20 to 600, preferably 50 to 500A. Such exterior dielectric layers typically form the initiallyuncovered exposed surface of the multi-layer coating which contacts theplasticized PVB sheet. These dielectrics are representative ofphotoreactive components with which the epoxy-modified sheet of theinvention is particularly usable. Exterior dielectric layers incomposition include those listed as spacer layers above. Preferreddielectric (photoreactive) layers as most frequently encountered incommercial structures are ZnO, chromium oxide (composed of CrO, CrO₂,CrO₃ and Cr₂ O₃), In₂ O₃, SnO and ITO.

The substrate for a heat-wave reflective and/or electrically conductivecoating comprises one or plural layers, one of which supports themulti-layer metal/dielectric coating in that a layer of the latter is inface-to-face contact with the substrate. The substrate can be any of avariety of materials. Usable substrates should not be prone to stretchto avoid cracking the metal/dielectric layers and should be free ofexcess volatiles such as plasticizers, water vapor or absorbed gases. Adielectric layer contacting the substrate should adhere well to thesubstrate. Dielectrics generally adhere well to glass, ceramics andcertain flexible plastics such as polyesters, cast acrylics,polycarbonates, chlorinated plastics and epoxies. Uncrosslinkedpolyurethanes and plasticized polyvinyl butyral substrates in directsupportive contact with the metal/dielectric stack are too soft andextensible. Preferred substrates are sheet(s) of transparent materialssuch as glass or non-extensible flexible thermoplastic materials such aslinear polyesters, e.g. polyethylene terephthalate (PET) (or equivalentmaterial having the characteristics of PET) which is commerciallyavailable as Mylar® or Hostaphan from Hoechst Celanese Corp. In twopreferred structures the layers of the metal/dielectric stack aresequentially magnetron-sputtered in one case on glass and another on aflexible substrate film of PET. In one preferred form, the glasssubstrate carrying the metal/dielectric stack is laminated to theepoxy-resin modified sheet of the invention (dielectric layer in contactwith one side of the sheet) to which a second glass layer is thenlaminated to the exposed side of the modified PVB sheet. In the otherpreferred structure, the PET film carrying the metal/dielectric stack isencapsulated within two layers of plasticized polyvinyl butyral, onelayer of which abuts the PET substrate and the other of which abuts thetop layer of the stack and is the epoxy-resin modified sheet of theinvention. The multi-layered sandwich containing plasticized PVB as theouter layers in the second preferred structure is then conventionallylaminated between two rigid members such as glass panes, oralternatively may be used as a bilayer structure by laminating it to onesuch rigid member intended to be the exterior side of a window.

Stability of the bond at the interface between theepoxy-resin-containing plasticized PVB sheet and the photoreactive,preferably anti-reflective, layer of a multi-layer metal/dielectricstack is measured by accelerated aging of the assembly by exposure to asource of intense radiation in the form of a Fadeometer (carbon arcsource), Weatherometer (xenon arc source) or equivalent system in whicha large percentage of emitted light is composed of UV radiation.Stability of the bond to light-induced, and more particularlyultraviolet light-induced, degradation as determined by the PummelAdhesion Test further described hereafter, is considered adequate if anassembly survives at least 1000 hours in such an accelerated agingsystem. This is about equal to one year of intense sunlight exposure asmight be encountered in Arizona, which in turn is representative of alonger period of exposure to less severe conditions.

The Pummel Adhesion Test was used to measure the impact-absorbing levelof interfacial adhesion of the PVB layer to a dielectric layer, usuallyof a multi-layered metal/dielectric stack with which it is in contact.Glass laminates containing the metal/dielectric stack (or in certaincases only a dielectric layer) and adhering plasticized PVB layer wereconditioned to 0° F. (-17° C.), pummeled with a 1 pound (454 g) hammerto break the glass and all broken glass unadhered to the PVB layerremoved. The amount of glass left adhered to the PVB layer is visuallycompared to a set of standards of known pummel scale, the higher thenumber of the standard, the more glass remaining adhered to theinterlayer--i.e. at a pummel of zero, no glass at all is left whereas ata pummel of 10, 100% of the interlayer surface is adhered to glass. Goodimpact dissipation is correlatable with a pummel adhesion value of 3.0to 7, preferably 4 to 6. At less than 3.0, too much glass is lost onimpact whereas at more than 7 adhesion is too high and impact strengthis poor.

The invention is further described with reference to the followingexemplary Example which is not intended to imply any limitation orrestriction on the invention. Unless otherwise indicated, parts andpercentages are by weight.

EXAMPLE A. Photoreactive Layer Adhesion

Samples were prepared of standard float glass (7.6×10×0.3 cm) sputtercoated using a Leybold Heraeus sputter coater in known manner with threesuccessive layers of zinc oxide dielectric and silver metal to provide aheat-wave-reflective multi-layer coating which, if operativelyassociated in known manner with bus bars in contact with the metal layer(see, for example, U.S. Pat. No. 4,782,216) would be electricallyconductive. The resulting structure and layer thicknesses were:glass/ZnO (400 A) (layer-1)/Ag (150 A) (layer 2)/ZnO (400 A) (layer 3).As reflected by the accelerated aging data for the control samplesbelow, the exposed ZnO dielectric top layer is the photoreactivecomponent in the context of the invention. In another instance a tenlayer heat-wave reflective coating on float glass was capped with about50A of uncovered TiOx where x was believed equal to 2. Further sampleswere prepared where single dielectric photoreactive layers of indiumoxide (In₂ O₃) of about 100 to 200 A thickness were sputter deposited onthe float glass to simulate expected performance if such single layerwas the top exposed layer of a multi-layer coating.

Epoxy resin (five parts per 100 parts PVB resin=5 phr) was dissolved indihexyl adipate plasticizer (32 parts plasticizer per 100 parts resin)which was then mixed for 7 min with PVB resin having 20.4% vinyl alcoholgroups in a Brabender mixer operating at 190° C. Tinuvin 328 (0.3-0.5pph on PVB resin) was included as UV stabilizer. 1280 ppm (resin)magnesium di(2-ethylbutyrate) was used for glass adhesion control. Aftercooling to room temperature, 0.76 mm sheets were compression molded fromthe mixtures using heated (177° C.) platens at 4000-5000 psi(27,560-34,450 kPa). The plasticized PVB sheet samples were laminated tothe exposed photoreactive dielectric layers described above usingstandard laminating conditions of about 143° C., 1275 kPa. Then a secondlayer of float glass was laminated to the exposed surface of theplasticized PVB sheet (with "air" side of glass against the sheet).Epoxy resins used are identified as follows:

    ______________________________________                                        Code Identification       Source                                              ______________________________________                                        E1   Diglycidyl ether of bisphenol A                                                                    Dow Chemical                                                                  DER 331                                             E2   Diglycidyl ether of bisphenol-F                                                                    Shell Chemical                                                                EPON Resin DPL-862                                  E3   Hydrogenated diglycidyl ether of                                                                   Shell Chemical                                           bisphenol A          EPONEX Resin 1510                                   E4   Polyepoxidized phenol                                                                              Dow Chemical                                             formaldehyde novolac DEN 431                                             E5   Diepoxide of poly(oxypropylene)                                                                    Dow Chemical                                             glycol               DER 732                                             ______________________________________                                    

The laminate samples were placed in a xenon arc Weatherometer with themetal/dielectric stack facing the xenon arc and above the plasticizedPVB sheet. Pummel adhesion (PA) was measured with time on the layer ofglass on which the metal and/or metal oxide was deposited, with resultsin Table 1 following. In all cases PA of the controls (no epoxy) on theglass layer just described, measured immediately after forming thelaminates (i.e. zero time in the Weatherometer), was acceptable andtherefore not included in the Table, which meant the dielectric layerhad not yet photoreactively degraded the bond with the PVB sheet. A PAof between 3 and 7 was acceptable and this is presented in the Table bylisting (in parenthesis) the number of accelerated exposure hours afterwhich this acceptable PA was measured. A PA less than 3 is unacceptableand such samples are listed as "F" (for "failed") followed by the numberof hours during which that sample had been exposed before theunacceptable PA was measured.

                  TABLE 1                                                         ______________________________________                                        Identity                                                                      Epoxy      Identity of Photoreactive Layer                                    In Sheet   ZnO         In.sub.2 O.sub.3                                                                         T.sub.i O.sub.2                             ______________________________________                                        None       F(2000)     F(100)     F(500)                                      E1         3(2000)     F(1000)    F(2000)                                     E2         3(2000)     4(2000)    F(2000)                                     E3         F(2000)     F(1000)    3(1000)                                     E4         5(2000)     4(1000)    F(500)                                      E5         4(2000)     4(2000)    3(2000)                                     ______________________________________                                    

The above results show that after at least 1000 hours acceleratedexposure in a Weatherometer the presence of 5 phr epoxy resin in PVBsheet improves the adhesion stability of the sheet to a variety ofphotoreactive metal/dielectric stacks or dielectric coatings, incomparison with such adhesion without the epoxy resin presence. Variousepoxy resins can be used depending on the identity of the contactingphotoreactive layer. From the above data, E5 (diepoxide ofpoly(oxypropylene) glycol) is a preferred material.

B. Glass Adhesion

The sheet samples described above were screened for possibleinterference by the epoxy resin with adhesion to glass, such glassadhesion being regulated by the glass adhesion-reducing agent in thesheet formulation. PA's of the second glass layer of the two glass layerlaminates described in Section A above (i.e. the uncoated glass layer indirect contact with the PVB sheet) were as follows:

    ______________________________________                                        Epoxy In Sheet                                                                              Pummel Adhesion*                                                ______________________________________                                        None (control)                                                                              5.4                                                             E1            5.0                                                             E2            5.4                                                             E3            4.8                                                             E4            6.0                                                             E5            4.8                                                             ______________________________________                                         *Average value obtained from five samples                                

The above illustrates that sheet with the noted epoxies comparesfavorably with the control. This shows these epoxies to be substantiallyinert to the level of adhesion to glass of sheet containing such epoxiesas regulated by the noted glass adhesion control agent.

The preceding description is for illustration only and is not to betaken in a limited sense. Various modifications and alterations will bereadily suggested to persons skilled in the art. It is intended,therefore, that the foregoing be considered as exemplary only and thatthe scope of the invention be ascertained from the following claims.

I claim:
 1. Plasticized polyvinyl butyral sheet containing epoxy resinbonded to a photoreactive layer of a heat-wave-reflective orelectrically conductive coating, the epoxy resin present in amounteffective to counteract reduction of adhesion at the bond interfaceafter prolonged exposure to light.
 2. The structure of claim 1 whenlaminated with glass exhibiting a pummel adhesion of at least 3.0 afterat least 1000 hours exposure to light which includes ultravioletradiation in a Fadeometer or Weatherometer.
 3. The structure of claim 2wherein the photoreactive layer is a dielectric material.
 4. Thestructure of claim 3 wherein the dielectric material is ZnO, In₂ O₃ orTiO₂.
 5. The structure of claim 4 wherein the dielectric material is ZnOor In₂ O₃.
 6. The structure of any of claims 1, 2, 3, 4 or 5 wherein theepoxy resin is selected from the group consisting of (a) epoxy resinscomprised of the monomeric diglycidyl ether of bisphenol-A; (b) epoxyresins comprised of the monomeric diglycidyl ether of bisphenol-F; (c)epoxy resins comprised of the hydrogenated diglycidyl ether ofbisphenol-A; (d) polyepoxidized phenol novolacs; (e) diepoxides ofpolyglycols; and (f) a mixture of any of the foregoing epoxy resins of(a) through (e).
 7. The structure of claim 6 wherein the epoxy resin isa diepoxide of poly(oxypropylene) glycol.
 8. The structure of claim 7wherein the amount of epoxy resin is about 3 to about 10 parts per 100parts of polyvinyl butyral.
 9. A prelaminate for a glazing panel havingimproved resistance to light exposure comprising two plasticizedpolyvinyl butyral sheets and an interposed intermediate layer supportedon a thermoplastic substrate, the intermediate layer containing aphotoreactive layer bonded to one of the plasticized polyvinyl butyralsheets, the plasticized polyvinyl butyral sheet bonded to thephotoreactive layer comprising one or more epoxy resins in amounteffective to counteract reduction of adhesion to the photoreactive layerafter prolonged exposure to light.
 10. The prelaminate of claim 9wherein the photoreactive layer is a dielectric material.
 11. Theprelaminate of claim 10 wherein the dielectric material is ZnO, In₂ O₃or TiO₂.
 12. The prelaminate of any of claims 9, 10, or 11 wherein theepoxy resin is selected from the group consisting of (a) epoxy resinscomprised of the monomeric diglycidyl ether of bisphenol-A; (b) epoxyresins comprised of the monomeric diglycidyl ether of bisphenol-F; (c)epoxy resins comprised of the hydrogenated diglycidyl ether ofbisphenol-A; (d) polyepoxidized phenol novolacs; (e) diepoxides ofpolyglycols; and (f) a mixture of any of the foregoing epoxy resins of(a) through (e).
 13. The prelaminate of claim 12 wherein the epoxy resinis a diepoxide of poly(oxypropylene) glycol.
 14. The prelaminate ofclaim 13 wherein said prolonged exposure to light is measured by atleast 1000 hours in a Weatherometer or Fadeometer.
 15. A prelaminate fora glazing panel having improved resistance to light exposure comprisingtwo outer plasticized polyvinyl butyral sheets bonded to an intermediatelayer interposed therebetween, said intermediate layer comprisingpolyethylene terephthalate film supporting a heat-wave-reflective orelectrically conductive multi-layer coating which includes a dielectriclayer interfacially bonded to one of the outer plasticized polyvinylbutyral sheets, said polyvinyl butyral sheet bonded to the dielectriclayer containing about 3 to about 10 parts per 100 parts of polyvinylbutyral of epoxy resin capable of counteracting the reduction ofadhesion at the bond interface that would occur in the absence of theepoxy resin after prolonged exposure to light.
 16. The prelaminate ofclaim 15 wherein the dielectric layer is ZnO, In₂ O₃ or TiO₂.
 17. Theprelaminate of either of claims 15 or 16 wherein the epoxy resin isselected from the group consisting of (a) epoxy resins comprised of themonomeric diglycidyl ether of bisphenol-A; (b) epoxy resins comprised ofthe monomeric diglycidyl ether of bisphenol-F; (c) epoxy resinscomprised of the hydrogenated diglycidyl ether of bisphenol-A; (d)polyepoxidized phenol novolacs; (e) diepoxides of polyglycols; and (f) amixture of any of the foregoing epoxy resins of (a) through (e).
 18. Theprelaminate of claim 17 wherein the epoxy resin is a diepoxide ofpoly(oxypropylene) glycol.
 19. The prelaminate of claim 18 wherein saidprolonged exposure to light is measured by at least 1000 hours in aWeatherometer or Fadeometer.